The activation and proliferation of T lymphocytes was studied in vitro to investigate the biochemical events associated with signals transmitted through specific membrane receptors. The expression of the cell cycle regulating kinase, cdc2, is linked to the T cell receptor on resting murine T lymphocytes. By the late G, phase, the kinase protein is detectable but does not express enzymatic activity until the cells reach premetaphase. As in other cell systems, this kinase activity is associated with the dephosphorylation of tyrosine residues within the cdc2 protein and one of the targets for phosphorylation is Hl histone. The proliferation of T lymphocytes as regulated by the interaction of IL-2 with its' specific receptor involves the phosphorylation state of tyrosine residues in a series of proteins. Withdrawal of IL-2 from a dependent T cell line results in apoptosis and the dephosphorylation of tyrosine residues in four distinct proteins but no apparent change in the phosphorylation state of cdc2. The IL-2 starved cells showed an unexpected significant increase in cdc2 specific MRNA, high levels of both Hl histone and case in kinase and accumulated in mitosis. The addiction of IL-2 returns the starved cells to a normal proliferative cycle and re-establishes the phosphorylation of the tyrosine residues of the affected proteins. In vivo experiments to demonstrate the effect of IL-2 as an limmunoadjuvant with classical influenza vaccination of old mice showed a very striking increase in serum antibody titer. The most striking observation was that the secondary response to the influenza vaccine was significantly enhanced irrespective of the presence of rIL-2 in the initial injection.